Supply Cable No Earth

[ChloeRadshaw1]

I had an electrical check who reported that the "Ze was too high" and that we should contact the DNO which is uk power netowkrs

The guy came out and basically said there was nothing he can do = He indicated that the best course of action was to install a RCD

Can anyone confirm if there is an easier way of correcting this and what the report is saying ?

 

[chivers67]

Sounds like You Need an Earth Rod installed in the ground if your system is TT plus an RCD unless the local DNO will provide you a PME. Can you post a photo of your incoming supply? You out in the country

 

[JohnW2]

.... unless the local DNO will provide you a PME. Can you post a photo of your incoming supply?

The note from UK Power Networks says that they would provide PME 'at the customer's cost'. On the face of it, presuming that the local supply network does indeed have PME, it's difficult to see what significant cost would be involved. If they decided that a new cutout was needed, I would have thought that would be their problem - I've never heard of a cistomer being charged for a cutout replacement!

However, it sounds as if this is a failed TN-S earth. If that is the case, then I thought that the DNO would (having previously supplied an earth) be under an obligation to maintain an adequate earth connection (of some sort), at their own cost?

Kind Regards, John

 

[flameport]

Despite what it says on the sheet, the 'lead sheath' is actually acting as a good earth - 6.65 ohms is far less (which is better) that any earth rod shoved in the ground will be.
The failure is probably at a joint box underground where the lead is connected to something else, probably an ancient cast iron box which has rusted.

options:
1. RCD and use the existing lead sheath as the earth electrode.
2. Have PME installed and use that.
3. Waste years of your life arguing for them to fix the TN-S earth and eventually give up and go for option 1 or 2.

He indicated that the best course of action was to install a RCD

They are correct. Option 1 would be quick and easy, and minimal cost. Should be done as soon as possible - assuming the installation doesn't already have RCD(s). If it does, then it could be that nothing is required.
Nothing wrong the the PME option, but it could be weeks/months before that can be done, assuming it is really an option at all.

I thought that the DNO would (having previously supplied an earth) be under an obligation to maintain an adequate earth connection (of some sort), at their own cost?

Theoretically - yes.
In reality - not going to happen.

 

[JohnW2]

Nothing wrong the the PME option, but it could be weeks/months before that can be done, assuming it is really an option at all.

As I implied, does not the fact that they suggested it as an option imply that the local network is probably already PMEd and, if so, as I sort-of asked before, why would it be more than trivial (and quick) for them to provide the OP with a PME earth?

Even if the existing cutout is not one designed for PME, can they not supply a neutral-derived 'PME' earth using a Henley or suchlike? ... or don't they do that (and, if not, why not?)

Kind Regards, John

 

[JohnW2]

options: 1. RCD and use the existing lead sheath as the earth electrode.

If the sheath has already got up to nearly 7Ω, is it not probable that it will progressively deteriorate to the point that one of the other options will then have to be adopted?

Kind Regards, John

 

[flameport]

It may deteriorate, but even if it does, many meters or 10s of meters of lead covered cable buried in the ground will still be orders of magnitude better than a ~1m piece of thin copper plated steel rod.

 

[JohnW2]

It may deteriorate, but even if it does, many meters or 10s of meters of lead covered cable buried in the ground will still be orders of magnitude better than a ~1m piece of thin copper plated steel rod.

Does that not depend upon how close the problem is to the OP's house?

Kind Regards, John

 

[securespark]

Post a photo of the cable where it comes into the building and the equipment attached to it.

If the DNO has provided an earth terminal in the past, it has a duty under ESQCR 2002 to maintain it at their cost.

options:
3. Waste years of your life arguing for them to fix the TN-S earth and eventually give up and go for option 1 or 2.

As a spark, I have regularly come across poor earth situations on TN supplies.

I have NEVER had to argue with a DNO to get what I have requested. And in most cases, I have had same day service!

In one case in Macc, the DNO had to dig up the customer's rather long driveway to replace a piece of rotten PILC cable, which had caused a high Ze.

Despite what it says on the sheet, the 'lead sheath' is actually acting as a good earth

But not as good as the 0.8 it should be, assuming TN-S.*

*The figures quoted in the regs can be exceeded by the DNO in certain situations, eg where supplies are rural.

EDITED FOR CLARITY

 

[JohnW2]

Post a photo of the cable where it comes into the building and the equipment attached to it.

I think that's important, because .....

Despite what it says on the sheet, the 'lead sheath' is actually acting as a good earth - 6.65 ohms is far less (which is better) that any earth rod shoved in the ground will be.

If the DNO has provided an earth terminal in the past, it has a duty under ESQCR 2002 to maintain it at their cost. .....
But not as good as the 0.8 it should be, assuming TN-S.

Quite so - and not good enough to provide ADS via OPDs - hence the suggestion that the OP should "install an RCD" (although many would argue that a single RCD covering the entire installation would be non-compliant with BS 7671).

However, I've just re-read the UPN note, and wonder if we have all been jumping to the wrong conclusion. It says "There is no approved earth connection at the property. Test show high ELI 6.65 ohm phase to lead sheath. Therefore the lead sheath is not a good earth".

Does this ("no approved earth connection") perhaps mean that there has never been a TN earth and that, when the guy measured the sheath to see if he could connect it as a TN-S earth he found that it had too high an impedance for that?

Kind Regards, John

 

[Lectrician]

They will usually provide PME for free and earth their sheath down at the cutout at the same time, as they don’t want a high resistance sheath on their network. Certainly my DNO anyway.

If you request a PME connection without a fault like this, it is chargeable, but it’s small money. You leave an earth ready to connect, they come and connect it.

A PME connection would be better than going TT.

 

[securespark]

Does this ("no approved earth connection") perhaps mean that there has never been a TN earth and that, when the guy measured the sheath to see if he could connect it as a TN-S earth he found that it had too high an impedance for that?

This is where photos will come in very handy.

AFAIK, there are no subterranean TT supplies in London, although I will bow to superior knowledge. Subterranean TT supplies are the only incoming supplies (to my knowledge) where you may encounter PILC with an unconnected lead sheath. All other PILC is either TN-S or ex-TN-S converted to TN-C-S.

So, unless it is TT using PILC, it is likely the cable should be maintained by the DNO.

However, I've just re-read the UPN note, and wonder if we have all been jumping to the wrong conclusion. It says "There is no approved earth connection at the property. Test show high ELI 6.65 ohm phase to lead sheath. Therefore the lead sheath is not a good earth".

The only other possibility for a get-out clause is to interpret "there is no approved earth connection at the property" to mean that, actually, there is one and it may well have been provided by the DNO, but it is made with a Tenby type clamp, which is not "approved".

In my experience, these have been used by several DNO's in the past (despite their rules forbidding it) to get a better connection to the lead sheath of PILC. They have also been used by DIY-ers and pseudo sparks which may explain UPN's reluctance to accept responsibility for it.

Having said all that, in my experience of working in the North West (Norweb), Merseyside/ North Wales/ Cheshire (Manweb), Staffs/ West Mids/ Worcestershire (MEB), Bristol (SWEB), the Yorkshires (YEB), Derbyshire, Notts and Leicestershire (EMEB) and Hants (SEB), I can't remember anyone from the DNO who has been obstructive regarding incoming supply earthing faults.
They have always gone out of their way to be helpful & I have only once or twice in all my career been aware of a charge being levied for supply of an earth terminal on an incoming supply.

 

[securespark]

So....what I would do if I were the OP would be to ring the number that the guy filling out the form helpfully didn't include and ask them to clarify their position, assuming (once we get photos) that we are able to rule out the possibility of a TT supply. Ask to speak to the guy who visited as anyone else won't know the score.

 

[JohnW2]

So....what I would do if I were the OP would be to ring the number that the guy filling out the form helpfully didn't include and ask them to clarify their position, assuming (once we get photos) that we are able to rule out the possibility of a TT supply. Ask to speak to the guy who visited as anyone else won't know the score.

I agree that clarification from the DNO is required - but I suspect that it may be a bit of a struggle to get that clarification. As I said before, we certainly could do with seeing some photos.

I was not thinking so much of it being a TT installation, since what the UPN guy wrote would really make little sense if that were the case - and the most he could really have commented on in that situation was the absence of (I presume) RCD(s). I was therefore wondering if there is, in fact, currently no earthing arrangement (TN or TT) at all.

If a TN-S earth were connected to the installation's MET, but had too high an impedance, I would have expected the note to have been written in terms of an "inadequate" earth connection, or something like that, rather than "no approved earth connection" - and one would certainly have expected that if such were the case, the DNO guy would know that it was their (the DNO's) responsibility to maintain (some sort of) adequate earth connection - rather than what he wrote.

My experience is obviously very limited but, consistent with what you have said about yours, over the years I have known of several people who have had their installations changed to TN-C-S because of a 'failed' TN-S earth, and on all occasions that has been done without argument, without charge and very quickly (within 24-48 hours of the issue having been reported).

My main point for the OP was that flameport's Option (1), plus RCD(s), may not really be applicable if there is no connection (and, particularly if there has never been a connection) between the sheath of the incoming supply cable and the installation's MET, and I would think it would be rather naughty ('not allowed') for an electrician to create such a connection.

Kind Regards, John

 

[securespark]

This is why I think he is describing it as "no approved earth connection"....because there is one, but it is not to standard, possibly because of an earth clamp. The pictures will tell, anyway.